The present invention relates to a decoding method and a communication terminal apparatus, and more particularly, to a decoding method and a communication terminal apparatus whereby data is decoded in accordance with a Transport Format Combination Indicator (TFCI) transmitted from a base station apparatus and a TFCI determination result.
In third generation mobile communication system using a CDMA technology, the execution of variable rate transmission by which the data rate is changed for every TTI (Transmission Time Interval) unit is proposed. The determination of data rate in the receiving side by a TFCI (Transport Format Combination Indicator) which includes the transmission format information is also proposed. Moreover, TTI is a data transmission length specified for every channel, and such a length is either 1, 2, 4 or 8 frame(s).
Moreover, the transmission format of data (namely, block size of data and the number of blocks of data) are specified by a TFCI number. In other words, the data rate is specified by the TFCI number. According to the specification of the third generation mobile communication system specified by 3GPP, the TFCI is shown by one number from among 1024 numbers from the 0th through 1023rd, converted into a codeword corresponding to each number (hereinafter, a codeword corresponding to each number is referred to as xe2x80x9cTFCI coding sequencexe2x80x9d) and then transmitted.
In the receiving side which receives the TFCI, the correlation values between each of the 1024 TFCI coding sequences which are specified beforehand and the actually received TFCI are calculated by the decoding apparatus, and the number corresponding to the correlation value which is maximum among the 1024 calculated correlation values is determined as the received TFCI number. Then, decoding apparatus performs decoding of data based on transmission format specified by the determined TFCI number.
Among the 1024 TFCI numbers, normally about 10 numbers or at most 100 numbers are actually used in mobile communication system. Consequently, because the correlation values corresponding to all 1024 TFCI coding sequences are calculated in the aforementioned conventional decoding apparatus even it is not important to calculate the correlation values corresponding to all 1024 TFCI coding sequences, there is a problem that the processing amount and power consumption which are required to determine the TFCI number become large. Thus, when the aforementioned conventional decoding apparatus is built in a communication terminal which is driven by a battery, there is a problem that the using time of a communication terminal becomes short.
Moreover, when an error occurrs with TFCI due to the influence of noise and such in the propagation path, a case might result in the decoding apparatus where the correlation value calculated using a TFCI coding sequence having a close inter-code distance to a transmitted TFCI coding sequence becomes maximum. In the case when the TFCI number corresponding to TFCI coding sequence with a near inter-code distance is the TFCI number which is not actually used, there is a problem that this TFCI number which is not actually used is erroneously determined as the received TFCI number.
Because the data will be decoded by an erroneous transmission format when the TFCI number is erroneously determined, all the data of TTI in respect to which the transmission format is erroneously determined might be erroneously decoded, and hence, the error rate characteristics of the received data will be remarkably deteriorated.
It is an object of the present invention to provide a decoding apparatus and a decoding method that are capable to improve the TFCI determining precision while reducing the amount of processing and power consumption which are required to determine the TFCI.
In order to achieve such an object, in the present invention, TFCI is determined using only those actually used TFCI numbers as candidates from among a plurality of TFCI numbers. Therefore, it is possible to improve the TFCI determining precision while reducing the processing amount and power consumption that are required to determine the TFCI.